Thank you for your question. The best source of nutrition for your pet is a balanced, nutritionally complete, commercial diet that has been tested with AAFCO feeding trials. The label on the side of the bag will indicate whether or not the diet that you are feeding to your pet has passed these tests. Additional vitamin and mineral supplementation is not required when feeding a complete and balanced diet, and some supplements can be harmful.
Calcium supplementation is not a usual treatment for joint disease (arthritis). I would not advise feeding ground up eggshells to your pet. Treatment of “joint disease” will be based on diagnostic tests (which will usually include radiographs or x-rays and bloodwork). These tests will determine whether a calcium deficiency is the reason for the problem, and what the best treatment would be for your pet. Call with any further questions. Dr. Peter Helmer, 813 885 4477